I have an XML file in response
Well that’s what this API (and most REST APIs) is about, you ask for information and you get a XML with the information in it that you need to parse. If your are in Java you can use JAXRS to automatically convert those XMLs into Java objects (it’s possible there is JAXRS support for other languages too but I’m not sure). You have various examples of how those files looks like in http://platform.xwiki.org/xwiki/bin/view/Features/XWikiRESTfulAPI#HFormatsoffiles and a link to the reference xsd.
You can also get a JSON instead of a XML for most of those APIs (see http://platform.xwiki.org/xwiki/bin/view/Features/XWikiRESTfulAPI#HAccessingtheservice) if it’s easier to parse for you.